In the design and manufacture of interior recessed fluorescent lighting fixtures it has always been desirable to maintain a low profile or silhouette for that fixture in order that the fixture might be accommodated within plenum spaces above the ceiling which vary significantly in their height or distance below the permanent ceiling. In order to achieve this, as small as possible, height dimension for the luminaire housing it has been a common practice to mount the wire-way and ballast housing to the undersurface of the top wall of the luminaire housing and thereby dispose the ballast housing and wire-way within the confines of the luminaire housing walls. In some constructions the ballast housing is made a permanent part of the luminaire top wall with access being provided from either one or both of the top and bottom of the ballast housing. In other constructions the ballast housing, in its entirety, may be removably secured to the underside of the top wall as illustrated in U.S. Pat. No. 3,281,588 to Spinetta.
As also illustrated in the above referenced Spinetta patent it is desirable that the fluorescent lamps mounted within the fixture be located the same distance above the luminaire refractor in order to provide both balanced light distribution and uniform appearance across the face of the refractor when viewed from the room lighted by the luminaire. For example, should one of the lamps be closer to the refractor than the remainder in the lamp array a distinct bright line will be in evidence as the refractor is viewed from below because of the proximity of the one lamp to the refractor surface.
Applications for recessed interior fluorescent luminaires differ considerably and it is at times desirable in the same fixture dimensions to provide a luminaire containing two, three or four elngated fluorescent lamps. It is also desirable that the number of lamps employed in the fixture be uniformly spaced within the luminaire housing to provide both uniform light distribution and symmetrical refractor appearance. In prior aret installations, when a single fixture design was utilized for both three and four lamp installations, the ballast housing was generally provided along the center line of the longitudinal dimension of the luminaire. This construction was no problem for four lamp fixtures but when three lamp installations were required the center lamp would have to be located below the ballast housing thereby bringing its location closer to the luminaire refractor and providing, when lighted, a non uniform appearance across the refractor face.